The production of modem printed wiring boards ("PWBs") for developing the desired electrically conductive pattern onto plastic substrates clad with copper foil, is a complex process. In the process, electrical interconnections are made by selectively removing unwanted copper foil or adding additional copper to the plastic substrates. The process requires careful removal of excess copper, reagents remaining from the copper deposition, powder, dust, oxides, and other debris or residuals after each construction step. It is not unusual for the process to require 100 or more individual steps because of the complexity of electrical circuitry configurations.
The removal of residual copper is termed "microetching" to emphasize the controlled nature of the action, that is, careful, gentle removal, as contrasted with energetic, bulk removal of copper by use of highly active chemicals, sometimes under extreme conditions of temperature, pH and/or modes of application. The microetching must be combined with cleaning action for efficient and economical processing. Microetching copper is a critical step in manufacturing PWB's because it insures that plating or resist layers will adhere to the substrate each time they are applied. The rate of residual copper removal by application of the microetchant is termed "etch rate".
Alkali metal persulfates such as sodium persulfate (sodium peroxydisulfate) are known to provide a superior copper topography when used as microetchants and cleaners in PWB manufacture. The topography is characterized by a rougher metallurgical surface than results from use of other microetchants, thereby providing a sufficient number of keying sites to ensure good adhesion for plating, resist and lamination. The pH of the persulfate solution can be lowered and surface texture can be varied by the addition of a mineral acid to the solution; phosphoric acid produces a relatively smoother topography whereas sulfuric acid increases roughness. Typically, about 1-2 percent mineral acid by volume of persulfate solution is used. Persulfates also are effective at lower temperatures than other microetchants. For example, persulfates are effective at about 75.degree.-90.degree. F. whereas hydrogen peroxide/sulfuric acid solutions require temperatures of about 115.degree.-130.degree. F.
Despite such advantages, persulfate microetchants tend to decompose rapidly in the presence of copper and exhibit irregular etch rates. This requires frequent microetchant/cleaner bath monitoring and adjustment of oxidizer concentration, particularly as the persulfate nears depletion. To some extent the etch rate can be controlled by adjusting the bath temperature and/or speed of the conveyor transporting PWBs or precursor copper clad substrates through the bath. However, the inevitable consequences are substantial amounts of waste product due to irregular microetching/cleaning and the formation of undesirable copper surfaces. In addition, copper solubilized in the bath must be removed to avoid precipitation of copper sulfate. Although most of the copper may be reclaimed from the baths by electrowinning procedures in order to satisfy environmental regulations, such procedures are time consuming and costly.
Further, because of the difficulty of controlling etch rate when using persulfates, it is routine to prepare, apply and dispose of microetchant/cleaner baths in batch mode. For efficiency of operation and reasonable cost, including reduction of downtime for cleaning and/or replenishing baths, PWB fabricators wish to replace such batch operations with continuous processing, including systems for continuously delivering reagents to the microetchant/cleaner baths and for regulating strength and other bath conditions relative to etch rate.
Japanese patent 86/435 to Komatsu et al and assigned to Mitsubishi Gas Chemicals KK, granted Aug. 1, 1986 on application 81/173606 (filed Oct. 29, 1981 and published May 4, 1983), discloses the addition of sulfuric acid and/or bisulfate ions to sodium or potasssium persulfate solutions for use in pre-etching ("soft" etching) during the production of printed circuit boards, in place of ammonium persulfate or sodium/potassium persulfate alone. Sulfate ion can be supplied by sulfates as well as by sulfuric acid. The mole ratios and concentrations of reagents in the solutions are such as to equal or exceed the copper dissolution rate (etch rate) as compared with that achieved with ammonium persulfate. It has been observed that the compositions of the patent do not provide sufficient control of etch rate.
U.S. Pat. No. 3,373,114 to Grunwald and assigned to MacDermid Inc. discloses dry compositions for desmutting aluminum and aluminum alloys in preparation for subsequent metal finishing operations. The compositions comprise mixtures of sodium, potassium or ammonium persulfate and a soluble acid salt of an acid selected from acid salts of sodium, potassium and ammonium acid salts of sulfuric acid, in a mole ratio of 1:1 to 10:1. A typical composition is a dry mixture of ammonium persulfate and sodium bisulfate. Baths prepared from the dry mixtures must have a pH not exceeding 2.0 for success in desmutting aluminum. It has been determined that such compositions are not useful in the microetching and cleaning of copper clad PWB substrates because the high acidity and hygroscopic nature of the bisulfate renders the compositions unstable and diminishes etch rate control.